1) Field of the Invention
The present invention relates to an alternative communication path design in a switching destination at the time of occurrence of a failure in a communication path. More particularly, the invention relates to a method of and apparatus for determining an alternative communication path, which can shorten a recovery time of the communication path at the time of the occurrence of a failure more realistically.
2) Description of the Related Art
According to diversification of service and demand expansion in Internet, a communication traffic volume in backbone networks is increasing remarkably. For this reason, in backbone networks, a capacity is being enlarged and a speed is being heightened based on wavelength division multiplexing (WDM) technique.
In order to realize flexible control of a mesh type network and an efficient operation due to sharing of an auxiliary wavelength, optical cross connect (OXC) and optical add-drop multiplexer (OADM) are being developed. A new communication infrastructure and introduction of its service are expected.
In large-capacity WDM networks, as a number of services in a system become larger, damage due to the occurrence of a failure is more extensive. For this reason, development of advanced management systems which heightens reliability of the networks is an issue. Particularly techniques to which recover services quickly from link failures and network node failures using optical layers are important.
The inventors of the present invention are examining a preplan type failure recovery system which realizes quick recovery from failures in the WDM networks (see “A Study on Path Restoration Method Based on Pre-planned Configuration” by Yasuki FUJII, Keiji MIYAZAKI and Khohei ISEDA, “Technical Report of IEICE” TM2000-60, pp. 67 to 72, November 2000).
In the preplan type failure recovery system, failure information is posted from network nodes that detect a failure to adjacent network nodes successively in network nodes where alternative communication path information is preset (flooding). As a result, the network nodes switch the communication paths in parallel according to the set alternative communication path information. As a result, the time required for dynamically searching for an alternative communication path can be shortened, and thus high-speed service recovery can be expected.
Even if the parallel switching of the communication paths is enabled, however, when the time until the network node on the alternative communication path to be switched receives the post of a failure is long, the high-speed service recovery cannot be realized.
FIG. 19 is a diagram explaining the above conventional preplan type failure recovery system. The drawing illustrates a network which is based on an optical path where an optical signal is transmitted/received between terminal stations, and particularly illustrates the network using optical cross connect (OXC) for relay, in which a plurality of optical signals are multiplexed in an optical fiber using the WDM technique.
In FIG. 19, it is supposed that communication is carried out on a current communication path WP1 of network nodes 17, 10, 12 and 14 from a network node 1 to a network node 2. If a failure 11 occurs between the network nodes 10 and 12, for example, the network node 12 on a lower stream side detects the failure. Each network node includes an optical cross connect as an optical switch.
The optical cross connect has a function of adjusting an angle of a built-in mirror (not illustrated) so as to switch a connecting state between a port on an optical signal input section and a port on an optical signal output section.
The network node 12 which detects the failure transfers a failure notification message 13 including failure location information to the (adjacent) network node 14. The (adjacent) network node 14 further posts the message 13 to the (adjacent) network node 15. In such a manner, the message 13 is posted to the adjacent network nodes successively (flooding).
The (alternative communication path relay) network nodes 15 and 16 and the (communication path switching) network nodes 14 and 17 transfer the failure notification message to all the adjacent network nodes excluding the network nodes which receive the message only when they receive the failure notification message at the first time.
The communication path is switched from the current communication path WP1 into an alternative communication path SP1 according to the preset alternative communication path information.
When the alternative communication path relay network node or the communication path switching network node on the set alternative communication path is separated from the failure detected network node and thus takes a long time until reception of the failure notification message, this is a major factor in a delay of the communication path recovery.
In the prior art, therefore, an alternative communication path, where the time required for transferring the failure notification message from the network node which detects the failure to all the network nodes on the alternative communication path does not exceed a given upper limit time, is pre-searched at the time of designing the alternative communication path, and this alternative communication path is set in the network nodes (see Japanese Patent Application Laid-Open Nos. H3-241938, 2002-77244, and 2002-281068).
In the prior art, when a failure occurs, the communication path is switched from a current communication path into an alternative communication path, but the recovery of the communication path is occasionally delayed depending on selection of a switching network node on an upper stream side with respect to a failure location on the current communication path.
That is to say, the network node where the communication path is switched requires an initial setting step of adjusting an angle of the mirror roughly as initial setting, and a finely adjusting step of finely adjusting the angle of the mirror so that an input of an optical signal is received from the upper stream and the output becomes normal. When these steps are not successively executed from the switching network node on the upper stream side to the switching network node on the lower stream side on the alternative communication path, the alternative communication path is not opened to traffic.
How quickly the switching network node on the upper stream side is switched is, therefore, important to shortening of the recovery time.
In the prior art, however, the alternative communication path, where the time for transferring the failure notification message from the failure detected network node does not exceed the given upper limit time, is set in the network nodes. Since the switching network node on the upper stream side is not, however, always switched in the quickest manner, the recovery of the communication path is occasionally delayed.